1. Field of the Invention
The present invention relates to an improved process for producing sticky polymers.
2. Description of the Prior Art
The introduction of high activity Ziegler-Natta catalyst systems has led to the development of new polymerization processes based on gas phase reactors such as disclosed in U.S. Pat. No. 4,482,687 issued Nov. 13, 1984. These processes offer many advantages over bulk monomer slurry processes or solvent processes. They are more economical and inherently safer in that they eliminate the need to handle and recover large quantities of solvent while advantageously providing low pressure process operation.
The versatility of the gas phase fluid bed reactor has contributed to its rapid acceptance. Alpha olefins polymers produced in this type of reactor cover a wide range of density, molecular weight distribution and melt indexes. In fact new and better products have been synthesized in gas phase reactors because of the flexibility and adaptability of the gas phase reactor to a large spectrum of operating conditions.
The term "sticky polymer" is defined as a polymer which, although particulate at temperatures below the sticking or softening temperature, agglomerates at temperatures above the sticking or softening temperature. The term "sticking temperature", is defined here as the temperature at which fluidization ceases because of excessive agglomeration of particles in the bed. The agglomeration may be spontaneous or occur over a period of time.
A polymer may be inherently sticky because of its chemical or mechanical properties or pass through a sticky phase during the production cycle. Sticky polymers are also referred to as non free-flowing polymers because of their tendency to compact into agglomerates of much larger size than the original particles. Polymers of this type show acceptable fluidity in a gas-phase fluidized bed reactor; however, once motion ceases, the additional mechanical force provided by the fluidizing gas passing through the distributor plate is insufficient to break up the agglomerates which form and the bed will not refluidize. These polymers are classified as those which have a minimum bin opening for free flow at zero storage time of two feet and a minimum bin opening for free flow at storage times of greater than five minutes of 4 to 8 feet.
Sticky polymers can also be defined by their bulk flow properties. This is called the Flow Function. On a scale of zero to infinity, the Flow Function of free flowing materials such as dry sand is infinite. The Flow Function of free flowing polymers is about 4 to 10, while the Flow Function of non-free flowing or sticky polymers is about 1 to 3.
Although many variables influence the degree of stickiness of the resin, it is predominantly governed by the temperature and the crystallinity of the resin. Higher temperatures of the resin increase its stickiness while less crystalline products such as very low density polyethylene (VLDPE), ethylene/propylene polymethylene (EPM), ethylene/propylene diene polymethylene (EPDM), and polypropylene (PP) copolymers usually display a larger tendency to agglomerate to form larger particles.
Thus the prior art has attempted to produce polymers at temperatures below the softening temperature of the polymers. This is based primarily on the fact that operating at or above the softening temperature would cause serious agglomeration problems.
More recently, U.S. Pat. No. 4,994,534 issued Feb. 19, 1991 to Seung J. Rhee et al and which is assigned to a common assignee discloses a process for producing sticky polymers at polymerization reaction temperatures in excess of the softening temperature of the sticky polymers in a fluidized bed reactor catalyzed by a transition metal catalyst. Basically the process comprises conducting the polymerization reaction above the softening temperatures of the sticky polymers in the presence of about 0.3 to about 60 weight percent, based on the weight of the final product, of an inert particulate material having a mean particle size of from about 0.01 to about 10 microns whereby polymer agglomeration of the sticky polymers is maintained at a size suitable for continuously producing the sticky polymers.
Thus, the above identified patent teaches that the addition of sufficient quantities of appropriate inert particulate materials (fluidization aids) to the reactor permits production of inherently sticky polymers. The term "inherently sticky" is used because the neat polymer is sticky, but after it is contacted with fluidization aid the polymer does not behave as a sticky polymer. Fluidization aids include carbon black and other carbon materials, silica, clays, and other materials which are inert under reaction conditions to produce the polymer. Levels of fluidization aids ranging from 0.3 to 60 weight percent, based on the weight of the polymer plus fluidization aid, have been effective in maintaining reactor operability.
Polyolefin polymers made in gas-phase, fluidized bed reactors are produced catalytically. The active catalytic species are generally supported on a relatively inert material, although there may be chemical as well as physical interaction between the catalytic species and the support. A typical support is silica gel. The size of the silica particles is frequently between 10 and 100 microns, with a small fraction of particles having diameters outside this range. A typical silica support used to make polyolefin polymers is Davison Grade 955 (available from W. R. Grace & Co.). The average particle size (APS) of catalyst made from it, as measured with a Microtrac analyzer, is 43 microns and the particle size distribution is shown in Table 1. Typical catalytic species impregnated on the support for use as polymerization catalysts include Ziegler-Natta type catalysts which contain titanium or vanadium active sites, and chromium salts.
When sticky polymers are produced with a fluidization aid in a gas-phase, fluidized-bed reactor using Ziegler-Natta catalyst supported on a relatively small APS silica, several operability deficiencies are experienced. In particular, over a period of several hours to several days, sheets of polymer form on the walls of the reactor above the fluidized bed. These sheets are normally formed along the conical transition section between the lower section of the reactor (the reaction zone) and the upper section of the reactor (the expanded section). Sheets are also formed in the lower part of the expanded section. As the sheets grow in size, pieces of them fall into the fluidized bed. If the pieces are sufficiently large, they tend to block the discharge port, necessitating shutdown of the reactor.
Another deficiency is the presence of slight agglomeration in the fluidized bed. Agglomerates are here defined as having a size in excess of 0.132 in. (larger than alternate sieve designation No. 6). Agglomerates are sometimes larger than 1 in. Agglomerates, particularly those larger than 1/4 in., adversely affect fluidizability in the reactor and make resin handling downstream of the reactor difficult.
Another important deficiency is the need for high levels of fluidization aid, sometimes in excess of 50 wt % on a final product basis.
Accordingly it is a principal object of the present invention to continuously produce inherently sticky polymers such as ethylene propylene rubber (EPR) in a gas-phase, fluidized-bed reactor over long period of time with good reactor operability.
It is another object to produce sticky polymers which are substantially free of agglomerates.
A further object of the invention is to reduce the required amount of fluidization aid required for sticky polymer production while achieving the above objectives.
These and other objects will be apparent from the following description of the invention.